Hierarchical Nanostructures of Iron Phthalocyanine Nanowires Coated on Nickel Foam as Catalysts for the Oxygen Evolution Reaction

In this paper, iron phthalocyanine nanowires on a nickel foam (FePc@NF) composite catalyst were prepared by a facile solvothermal approach. The catalyst showed good electrochemical oxygen evolution performance. In 1.0 M KOH electrolyte, 289 mV low overpotential and 49.9 mV dec-1 Tafel slope were seen at a current density of 10 mA cm-2. The excellent electrochemical performance comes from the homogeneous dispersion of phthalocyanine nanostructures on the surface of the nickel foam, which avoids the common agglomeration problem of such catalysts and provides a large number of active sites for the OER reaction, thus improving the catalytic performance of the system.

Built-in electrophilic/nucleophilic domain of nitrogen-doped carbon nanofiber-confined Ni2P/Ni3N nanoparticles for efficient urea-containing water-splitting reactions

Transferring urea-containing waste water to clean hydrogen energy has received increasing attention, while challenges are still faced in the sluggish catalytic kinetics of urea oxidation. Herein, a novel hybrid catalyst of Ni2P/Ni3N embedded in nitrogen-doped carbon nanofiber (Ni2P/Ni3N/NCNF) is developed for energy-relevant urea-containing water-splitting reactions. The built-in electrophilic/nucleophilic domain resulting from the electron transfer from Ni2P to Ni3N accelerates the formation of high-valent active Ni species and promotes favourable urea molecule adsorption. A spectral study and theoretical analysis reveal that the negatively shifted Ni d-band centre in Ni2P/Ni3N/NCNF weakens the adsorption of intermediate CO2 and facilitates its desorption, thereby improving the urea oxidation reaction kinetics. The overall reaction process is also optimized by minimizing the energy barrier of the rate-determining step. Following the stability test, the surface reconstruction of the pre-catalyst is discussed, where an amorphous layer of NiOOH as the real active phase is formed on the surface/interface of Ni2P/Ni3N for urea oxidation. Benefiting from these characteristics, a high current density of 151.11 mA cm-2 at 1.54 V vs. RHE is obtained for urea oxidation catalysed by Ni2P/Ni3N/NCNF, exceeding that of most of the similar catalysts. A low cell voltage of 1.39 V is required to reach 10 mA cm-2 for urea electrolysis, which is about 200 mV less than that of the general water electrolysis. The current work will be helpful for the development of advanced catalysts and their application in the urea-containing waste water transfer to clean hydrogen energy.

A NiMOF integrated with conductive materials for efficient bifunctional electrocatalysis of urea oxidation and oxygen evolution reactions

The development of urea oxidation reaction (UOR) and oxygen evolution reaction (OER) bifunctional electrocatalysts has dual significance in promoting hydrogen energy production and urea-rich wastewater treatment. Herein, a carboxylated multi-walled carbon nanotube (MWCNT-COOH)-ferrocene carboxylic acid (Fc-COOH) modulated NiMOF hybrid material (MWCNT-NiMOF(Fc)) has been synthesized for dual electrocatalysis of the UOR and OER. The material characterization results indicated that MWCNT-COOH and Fc-COOH were integrated into the framework structure of the NiMOF. The direct interaction between the NiMOF and MWCNT/Fc facilitated electron transfer in the hybrid material and led to lattice strain, which improved the charge transfer kinetics, promoted the exposure of more unsaturated Ni sites, and increased the electrochemically active surface area. These factors together enhanced the electrocatalytic activity of MWCNT-NiMOF(Fc) towards the UOR and OER. Using a glassy carbon electrode as the substrate, MWCNT-NiMOF(Fc) exhibited low potential requirements, low Tafel slopes, and high stability. In overall urea and water splitting electrolysis cells, the excellent UOR and OER dual functional catalytic ability and enormous practical application potential of the MWCNT-NiMOF(Fc) modified foam nickel electrode were further demonstrated. On the basis of the above research, the influence of a KOH environment on urea electrolysis was further studied, and the urea electrolysis products were analyzed, promoting a more comprehensive understanding of the catalytic performance of MWCNT-NiMOF(Fc) for urea oxidation. This study provides a new approach for developing high-performance NiMOF-based electrocatalysts for challenging bifunctional UOR/OER applications, and has potential application value in hydrogen production from urea-containing wastewater electrolysis.

Mott Schottky CoSx-MoOx@NF heterojunctions electrode for H2 production and urea-rich wastewater purification

The sluggish kinetics of oxygen evolution reaction (OER) is the bottleneck of alkaline water electrolysis. The urea oxidation reaction (UOR) with much faster kinetics was to replace OER. To further promote UOR, a heterojunction structure assembled of CoS_x and MoO_x was established, and then its superior catalytic activity was predicted by DFT calculation. After that, an ultra-thin CoS_x-MoO_x@nickel foam (CoS_x-MoO_x@NF) electrode with a Mott-Schottky structure was prepared via a facile hydrothermal method, followed by a low-temperature vulcanization. Results highlighted CoS_x-MoO_x@NF electrode presented a superior performance toward UOR, OER, and H₂ evolution reaction (HER). Notably, it exhibited excellent electrocatalytic performance for OER (1.32 V vs. RHE, 10 mA cm^-2), UOR (1.305 V vs. RHE, 10 mA cm^-2), and urea-assisted overall water splitting with a low voltage (1.38 V, 10 mA cm^-2) when CoS_x-MoO_x@NF electrode served as both anode and cathode. It is promising to use CoS_x-MoO_x@NF in an electrochemical system integrated with H₂ generation and urea-rich wastewater purification.

Molybdenum carbide/Ni nanoparticles-incorporated carbon nanofibers as effective non-precious catalyst for urea electrooxidation reaction

In this study, molybdenum carbide and carbon were investigated as co-catalysts to enhance the nickel electro-activity toward urea oxidation. The proposed electrocatalyst has been formulated in the form of nanofibrous morphology to exploit the advantage of the large axial ratio. Typically, calcination of electropsun polymeric nanofibers composed of poly(vinyl alcohol), molybdenum chloride and nickel acetate under vacuum resulted in producing good morphology molybdenum carbide/Ni NPs-incorporated carbon nanofibers. Investigation on the composition and morphology of the proposed catalyst was achieved by XRD, SEM, XPS, elemental mapping and TEM analyses which concluded formation of molybdenum carbide and nickel nanoparticles embedded in a carbon nanofiber matrix. As an electrocatalyst for urea oxidation, the electrochemical measurements indicated that the proposed composite has a distinct activity when the molybdenum content is optimized. Typically, the nanofibers prepared from electrospun nanofibers containing 25 wt% molybdenum precursor with respect to nickel acetate revealed the best performance. Numerically, using 0.33 M urea in 1.0 M KOH, the obtained current densities were 15.5, 44.9, 52.6, 30.6, 87.9 and 17.6 mA/cm² for nanofibers prepared at 850 °C from electropsun mats containing 0, 5, 10, 15, 25 and 35 molybdenum chloride, respectively. Study the synthesis temperature of the proposed composite indicated that 1000 °C is the optimum calcination temperature. Kinetic studies indicated that electrooxidation reaction of urea does not follow Arrhenius's law.

Tragacanth Gum Hydrogel-Derived Trimetallic Nanoparticles Supported on Porous Carbon Catalyst for Urea Electrooxidation

The fabrication of electrocatalysts with high catalytic activity, high durability and low cost towards urea oxidation by a facile method is a great challenge. In this study, non-precious NiCoFe trimetallic supported on porous carbon (NiCoFe@PC) was prepared via gelation and pyrolysis method, presenting a remarkable electrocatalytic activity with low onset potential for urea oxidation in an alkaline medium. Field-emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) were used to clarify the morphology of the NiCoFe@PC nanostructure and its nanoparticle size of 17.77 nm. The prepared catalyst with the composition ratio of 24.67, 5.92 and 5.11% for Ni, Fe and Co, respectively, with highly crystalline nanoparticles, fixed on porous carbon, according to energy-dispersive X-ray (EDX) and X-ray diffraction (XRD) analysis. The FeCoNi@PC catalyst showed a catalytic activity of 44.65 mA/cm² at 0.57 V vs. Ag/AgCl and a low onset potential of 218 mV, which is superior to many other transition bi/trimetallic-based catalysts previously reported.

Ni2P nanoflakes for the high-performing urea oxidation reaction: linking active sites to a UOR mechanism

Urea electrolysis is regarded as an effective method for addressing both energy and environment issues. Herein, we successfully synthesized Ni2P nanoflakes for catalyzing the urea oxidation reaction (UOR). Due to the higher electrical conductivity as well as the prevailing tendency in triggering the UOR via a direct electro-oxidation mechanism, Ni2P nanoflakes exhibit comparable UOR activity (1.33 V vs. RHE for onset-potential, and 95.47 mA·cm-2 at 1.6 V vs. RHE) to the most active state-of-the-art catalysts, rendering them an effective alternative to precious metals such as Pt and Rh. The accelerated proton-coupled electron transfer (PCET) process caused by PO43- facilitates the in situ generation of NiOOH; thus, the UOR process is initiated at a lower onset-potential on Ni2P nanoflakes than on β-Ni(OH)2 nanoflakes. The in situ generated NiOOH instead of the Ni2P phase in Ni2P nanoflakes functions as an active site during the UOR process, while both NiOOH and the Ni2P phase serve as active sites in the OER process. This work provides insights into the understanding of the UOR mechanism and opens a new avenue to design low-cost Ni-based phosphide UOR catalysts.

Nickel phosphide nanoalloy catalyst for the selective deoxygenation of sulfoxides to sulfides under ambient H2 pressure

Exploring novel catalysis by less common, metal-non-metal nanoalloys is of great interest in organic synthesis. We herein report a titanium-dioxide-supported nickel phosphide nanoalloy (nano-Ni2P/TiO2) that exhibits high catalytic activity for the deoxygenation of sulfoxides. nano-Ni2P/TiO2 deoxygenated various sulfoxides to sulfides under 1 bar of H2, representing the first non-noble metal catalyst for sulfoxide deoxygenation under ambient H2 pressure. Spectroscopic analyses revealed that this high activity is due to cooperative catalysis by nano-Ni2P and TiO2.

Efficient nanointerface hybridization in a nickel/cobalt oxide nanorod bundle structure for urea electrolysis

Urea electrolysis has received great attention for the energy-relevant applications, and efficient nanostructured catalysts are required to overcome the sluggish urea oxidation kinetics. Herein, we noticed that the valence state of Ni in the hybrid Ni/Co oxide nanorods can be correlated to the catalytic capability for urea oxidation. Crystal lattice hybridization was found in the interface of Ni/Co oxide nanoparticles that assembled as a nanorod bundle structure. The more or the less of Ni2+/Ni3+ generated lower catalytic ability, and Ni/Co oxide with the optimum content of Ni2+/Ni3+ exhibited the highest catalytic ability for urea oxidation because of the efficient synergism, resulting from the formation of high valence state of Ni species and improved kinetics. A low onset potential of 1.29 V was required for the urea oxidation compared with the high onset potential of 1.52 V for water oxidation; high selectivity for urea oxidation was found in the potential below 1.50 V, and as a promising application for urea-assisted water electrolysis about 190 mV less was required to provide 10 mA cm-2 in the two-electrode system, indicating the energy-efficient nature for hydrogen evolution. The study provides some novel insights into the Ni/Co catalyst design and fabrication with efficient catalytic synergism for electrocatalysis.

Designing Advanced Catalysts for Energy Conversion Based on Urea Oxidation Reaction

Urea oxidation reaction (UOR) is the underlying reaction that determines the performance of modern urea-based energy conversion technologies. These technologies include electrocatalytic and photoelectrochemical urea splitting for hydrogen production and direct urea fuel cells as power engines. They have demonstrated great potentials as alternatives to current water splitting and hydrogen fuel cell systems with more favorable operating conditions and cost effectiveness. At the moment, UOR performance is mainly limited by the 6-electron transfer process. In this case, various material design and synthesis strategies have recently been reported to produce highly efficient UOR catalysts. The performance of these advanced catalysts is optimized by the modification of their structural and chemical properties, including porosity development, heterostructure construction, defect engineering, surface functionalization, and electronic structure modulation. Considering the rich progress in this field, the recent advances in the design and synthesis of UOR catalysts for urea electrolysis, photoelectrochemical urea splitting, and direct urea fuel cells are reviewed here. Particular attention is paid to those design concepts, which specifically target the characteristics of urea molecules. Moreover, challenges and prospects for the future development of urea-based energy conversion technologies and corresponding catalysts are also discussed.

Metal-organic framework-derived Ni@C and NiO@C as anode catalysts for urea fuel cells

Highly porous self-assembled nanostructured Ni@C and NiO@C were synthesized via calcination of a Ni-based metal–organic framework. The morphology, structure, and composition of as synthesized Ni@C and NiO@C were characterized by SEM, FIB-SEM, TEM, and XRD. The electro-catalytic activity of the Ni@C and NiO@C catalysts towards urea oxidation was investigated using cyclic voltammetry. It was found that the Ni@C had a higher residual carbon content and a higher specific surface area than NiO@C, thus exhibiting an enhanced electrochemical performance for urea oxidation. A direct urea fuel cell with Ni@C as an anode catalyst featured an excellent maximum power density of 13.8 mW cm⁻² with 0.33 M urea solution in 1 M KOH as fuel and humidified air as oxidant at 50 °C, additionally showing excellent stability during continuous 20-h operation. Thus, this work showed that the highly porous carbon-supported Ni catalysts derived from Ni-based metal–organic framework can be used for urea oxidation and as an efficient anode material for urea fuel cells.

Recent progress with electrocatalysts for urea electrolysis in alkaline media for energy-saving hydrogen production

Urea electrolysis is a promising energy-saving avenue for hydrogen production owing to the low cell voltage, wastewater remediation and abundant electrocatalysts.

Recent Trends in Synthesis and Investigation of Nickel Phosphide Compound/Hybrid-Based Electrocatalysts Towards Hydrogen Generation from Water Electrocatalysis

Sustainable and high performance energy devices such as solar cells, fuel cells, metal-air batteries, as well as alternative energy conversion and storage systems have been considered as promising technologies to meet the ever-growing demands for clean energy. Hydrogen evolution reaction (HER) is a crucial process for cost-effective hydrogen production; however, functional electrocatalysts are potentially desirable to expedite reaction kinetics and supply high energy density. Thus, the development of inexpensive and catalytically active electrocatalysts is one of the most significant and challenging issues in the field of electrochemical energy storage and conversion. Realizing that advanced nanomaterials could engender many advantageous chemical and physical properties over a wide scale, tremendous efforts have been devoted to the preparation of earth-abundant transition metals as electrocatalysts for HER in both acidic and alkaline environments because of their low processing costs, reasonable catalytic activities, and chemical stability. Among all transition metal-based catalysts, nickel compounds are the most widely investigated, and have exhibited pioneering performances in various electrochemical reactions. Heterostructured nickel phosphide (Ni_xP_y) based compounds were introduced as promising candidates of a new category, which often display chemical and electronic characteristics that are distinct from those of non-precious metals counterparts, hence providing an opportunity to construct new catalysts with an improved activity and stability. As a result, the library of Ni_xP_y catalysts has been enriched very rapidly, with the possibility of fine-tuning their surface adsorption properties through synergistic coupling with nearby elements or dopants as the basis of future practical implementation. The current review distils recent advancements in Ni_xP_y compounds/hybrids and their application for HER, with a robust emphasis on breakthroughs in composition refinement. Future perspectives for modulating the HER activity of Ni_xP_y compounds/hybrids, and the challenges that need to be overcome before their practical use in sustainable hydrogen production are also discussed.

Bonding state synergy of the NiF2/Ni2P hybrid with the co-existence of covalent and ionic bonds and the application of this hybrid as a robust catalyst for the energy-relevant electrooxidation of water and urea

Among the energy-relevant electrochemical reactions, the electrochemical water and urea oxidation reactions are very significant for solving the increasing energy crisis and environmental pollution. Herein, the NiF2/Ni2P hybrid catalyst, in which covalent and ionic bonds co-existed, was found to be a very active catalyst for these electrochemical reactions occuring during the electrolysis of water. The bonding states of the covalent and ionic bonds were verified by the crystal structure and surface chemical state revealed by spectral analysis. As a bifunctional catalyst for the electrooxidation of water and urea, the NiF2/Ni2P hybrid structure demonstrated higher catalytic activity, kinetics and stability in the catalytic reaction than the individual components NiF2 and Ni2P under the same conditions. Specifically, an overpotential as low as 283 mV could drive the benchmark current density of 10 mA cm-2 for the oxygen evolution reaction, significantly lower than the overpotential required for the NiF2 (393 mV) and Ni2P materials (342 mV); the maximum current density for urea electrooxidation could reach 157.35 mA cm-2 at 1.53 V, which was much higher than those of NiF2 (23.55 mA cm-2) and Ni2P (102.72 mA cm-2). The catalytic performance also outperformed those of the recently reported similar advanced catalysts, and the high performance could be attributed to the highly exposed active sites, rough surface area, excellent charge transfer ability, and especially, the synergistic effects between the covalent and ionic bonds in the catalyst system. Using a commercial Pt/C catalyst as a cathode, the cell potential for urea-assisted water electrolysis could be reduced to 1.5 V to obtain the current density of nearly 40 mA cm-2 in a two-electrode system (Pt/C||NiF2/Ni2P), about 300 mV less than that required for water electrolysis in the general alkaline electrolyte. The current study demonstrates the significance of bonding state synergy in an advanced catalyst for water electrolysis and sheds some light on catalyst development in energy chemistry.

Atomically thick Ni(OH)2 nanomeshes for urea electrooxidation

Atomically thick ultrathin nanomeshes (NMs) possessing the inherent advantages of both two-dimensional nanomaterials and porous nanomaterials are attracting increasing interest in catalysis and electrocatalysis. Herein, we report a direct chemical synthesis of atomically thick Ni(OH)2-NMs by a NaBH4 assisted cyanogel hydrolysis method, which overcomes the shortcoming of the post-etching method for NM synthesis. Various physical characterization methods show that the as-synthesized Ni(OH)2-NMs have 1.7 nm thickness, a big surface area, abundant nanoholes, and numerous surface/edge atoms with low-coordination numbers. The as-synthesized Ni(OH)2-NMs show a better electrocatalytic performance for the urea oxidation reaction than conventional Ni(OH)2 nanoparticles without holes in the alkaline electrolyte, including a lower onset oxidation potential, faster reaction kinetics, and higher mass activity.